(a) Field
The described technology relates generally to a method and an apparatus for transmitting frame, and more particularly, to a method and an apparatus for transmitting frame in a wireless local area network (WLAN).
(b) Description of the Related Art
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.”
After an original standard was published in 1999, new version standards are continuously published by amendments. The IEEE standard 802.11a (IEEE Std 802.11a-1999) supporting 2.4 GHz band was published in 1999, the IEEE standard 802.11b (IEEE Std 802.11b-1999) supporting 5 GHz band was published in 1999, and the IEEE standard 802.11g (IEEE Std 802.11g-2003) supporting 5 GHz band was published in 2003. These standards are called legacy. Subsequently, the IEEE standard 802.11n (IEEE Std 802.11n-2009) for enhancements for higher throughput (HT) was published in 2009, and the IEEE standard 802.11ac (IEEE 802.11ac-2013) for enhancements for very high throughput (VHT) was published in 2013. Recently, a high efficiency (HE) WLAN for enhancing the system throughput in high density scenarios is being developed by the IEEE 802.11ax task group.
Since a WLAN uses an unlicenced frequency bandwidth, various devices exist to cause interference. In order to prevent collision with other devices, a WLAN device uses a carrier sense multiple access/collision avoidance (CSMA/CA) scheme to perform communication only when a channel is not in use. Here, the WLAN device performs a clear channel assessment (CCA) to determine whether a channel is in use, and when the CCA value is higher than a threshold value, the WLAN device determines that a channel is in use and defers transmission to the corresponding channel.
In the IEEE 802.11n standard, different CCA methods are applied to a primary channel and a secondary channel. That is, a primary channel CCA uses a signal detection method through virtual carrier detection and a secondary channel CCA uses only an energy detection method. Here, CCA threshold values respectively set for valid 20 MHz and 40 MHz bandwidth of a primary channel are −82 dBm and −79 dBm, and a CCA threshold value set for a secondary channel is −62 dBm. The difference between the CCA threshold values of the primary channel and the secondary channel causes unfairness between the primary channel and the secondary channel. Also, since the CCA threshold value of the secondary channel is higher than that of the primary channel, there is a higher possibility of collision in the secondary channel than in the primary channel.
The IEEE 802.11ac standard supports 80 MHz or 160 MHz, wider bandwidth than that of the IEEE 802.11n standard, and thus, a wireless communication network of the IEEE 802.11ac standard includes a larger number of secondary channels than that of the IEEE 802.11n standard. Thus, in order to increase CCA performance of the secondary channel, the IEEE 802.11 standard has adopted a guard interval (GI) correlation-based CCA. Since a GI may be formed by a cyclic prefix (CP), the GI correlation may be termed a CP correlation. The CP correlation-based CCA is a method for determining whether a channel is in use on the basis of a CP correlation, in which a −72 dBm threshold value is used for every 20 MHz channel. Thus, the IEEE 802.11ac standard may increase CCA sensitivity of a secondary channel to −72 dBm.
The IEEE 802.11ax task group is developing a high efficiency WLAN (HEW) robust to delay spread and supporting high aggregate throughput. To this end, the IEEE 802.11ax uses a larger number of subcarriers by applying 128 or 256-point FET, instead of 64-point FFT previously used in the 20 MHz bandwidth. Also, the IEEE 802.11ax uses a longer CP by two or four times than existing 0.8 us.
A VHT device supporting the IEEE 802.11ac standard is implemented to calculate a CP correlation in symbol unit of 4 us/3.6 us, and thus, if a length of the received symbol is different from 4 us/3.6 us, it is difficult to calculate the CP correlation. In a HEW environment in which the HEW device supporting the IEEE 802.11ax and the VHT device coexist, if the VHT device cannot calculate the CP correlation of a frame transmitted from the HEW device, it will lead to difficulty in using a CP correlation-based CCA in a secondary channel.